1. Field of the Invention
This invention relates to reflection Bragg Gratings and more particularly to a system and a method for recording large-sized reflection Bragg Gratings and a system and a method for apodization of large-sized reflection Bragg Gratings.
2. Description of Related Art
Reflection Bragg Gratings are volume gratings that are written by recording volume holograms created by the interference between two optical beams. Writing the volume holograms may be performed through transmission geometry or reflection geometry setups.
FIG. 1 is a schematic plan view of a conventional reflection Bragg Grating recording system in transmission geometry. When recording the reflection Bragg Grating in transmission geometry, the two optical beams 1, 2 are incident upon the same face of a recording medium 3 and transmitted through the recording medium 3. The recording medium 3 may also be referred to as a target or a sample and it may be made from a material such as lithium niobate or photothermorefractive glass.
FIG. 2 shows a schematic plan view of a conventional reflection Bragg Grating recording system in reflection geometry. When recording the reflection Bragg Grating in reflection geometry, two optical beams 10, 20 are incident on opposite faces of a recording medium 30 and transmitted through the recording medium 30. The interference pattern caused by the two beams 10, 20 intersecting at the center of the recording medium 30 is recorded. Each beam enters the recording medium at the same angle of incidence, interferes with the other beam, and continues through the medium to exit from the opposite face. The interference patterns are used to record the volume holograms within the medium.
There are a number of problems associated with recording large-sized reflection Bragg gratings. For example, even highly expanded Gaussian beams, usually used for recording well-characterized gratings, cannot provide high-homogeneous distribution of exposure dosage across a large area. This results in non-uniform diffraction efficiency (DE) distribution across recorded gratings. Small-scale distortions in the spatial distribution of the interfering beams are imprinted in the gratings during recording and result in hologram degradations. In the case when the Bragg Gratings are written in transmission geometry and are read in reflection geometry, large-size grating recording is available only for rather low absorbing materials. High absorption material, such as photothermorefractive glass, prevent the deep penetration of laser beams into the material and make recording of large-sized holograms difficult.
Referring to FIGS. 1 and 2, the incident optical beams have a large size to allow recording over a large area of incidence. Usually, the beams used for hologram recording have a Gaussian spatial profile and a wide distribution in intensity. Therefore, the recorded interference patterns have an uneven modulation, causing the exposure dosage across a large area of the medium to have a nonhomogenous distribution. The diffraction efficiency across recorded gratings, therefore, may have a nonuniform distribution.
To provide approximately the same exposure dosage over a recording medium, only the central portions of the Gaussian beams are typically used for recording. This necessitates obtaining a large interference area. To obtain a large interference area, expanded Gaussian beams are created by expanding the output of well-characterized, single-transverse-mode lasers with diameters from 1-2 mm to several tens of cms.
Use of such expanded beams, however, presents additional problems. One problem is that expansion of the beams requires large, expensive optics. Additionally, even using the highly expanded Gaussian beams, there is still a considerable difference in the exposure dosage of different parts of the holograms. A large expansion is also difficult to implement without truncating the edges of the expanded beams. The beam truncation causes diffraction, which manifests itself as additional patterns in the hologram. These parasitic patterns modulate the dominant grating of the hologram and reduce the overall performance of the final product. Further, truncating the edges of the expanded beams decreases the power density of the interfering beams, which necessitates undesirably lengthy exposures. Another problem with typical systems used to record large-sized reflection Bragg Gratings is that distortions in the spatial distribution of the interfering beams result in hologram degradations. These distortions may be caused by imperfections of optics, diffraction due to dust particles or inhomogeneities of optics, and interference between the main beam and the beams re-reflected from the different surfaces of the optical set up and recording media.
Typical systems for recording Bragg Gratings include wavefront-splitting interferometers, phase-masks, or amplitude-splitting interferometers. The wavefront-splitting interferometer systems carve out two interfering beams from different areas of the wavefront of a spatially coherent beam. Such splitting, however, results in diffraction at the boundary of the cut, causing the parasitic interference fringes described above. Further, additional beam expansion is necessary if large-sized gratings are to be recorded. In phase-mask systems, a phase mask is illuminated by a single laser beam, creating interfering beams on a closely positioned target. Large-sized or thick gratings can therefore not be recorded using these systems. In amplitude-splitting interferometer systems, two interfering beams are created by splitting a parent beam in two, and combining the two beams on a target in transmission or reflection geometry.
The basic maximum in the spectral or angular distribution of diffraction efficiency in finite length Bragg Gratings with uniform modulation of the refractive index is typically accompanied by a number of sidelobes at adjacent wavelengths or angles. Refractive index or index of refraction of a material is the factor by which the phase velocity of electromagnetic radiation is slowed in that material, relative to its velocity in a vacuum. Apodization, i.e., elimination of these sidelobe reflections, is desirable or even necessary in some applications. For example, in dense wavelength division multiplexing (DWDM), the apodization of Bragg Gratings is necessary to exclude crosstalk between information channels. Another example is the use of Bragg Gratings as spectral or angular selectors in laser techniques when only the main diffraction maximum is needed for a proper spectral or angular shape of laser radiation.
The conventional methods of reflection Bragg Grating apodization are applicable only to fiber gratings or to small-sized gratings (i.e., gratings small in the direction perpendicular to the direction of beam propagation). In these cases, the apodization is produced from exposure along the lateral sides of the Bragg Gratings.
In another method, the produced Bragg grating is irradiated with a highly absorbed radiation from both sides. The beams are absorbed in a photosensitive medium such that the sides of the material are provided with a large exposure dosage, which rapidly decreases before reaching the center. This results in the more efficient partial “erasing” of recorded grating close to the surfaces. These apodization methods thus result in a nonuniform average dosage of irradiation and in a nonuniform average refractive index through the thickness of the material, which causes a shift in the frequency of light reflected from the surface regions of Bragg Grating relative to the central part when it is read.